The Science of Cleaning

The Science of Cleaning

Solubility: The key to effective cleaning

Water has some very strange chemical and physical properties. A unique property of water is its ability to dissolve a large variety of chemical substances. It dissolves salts and other ionic compounds, as well as polar covalent compounds such as alcohols and organic acids. In fact, water is sometimes called the universal solvent because it can absorb so many things.

However, water is restricted in its ability to clean because water and oil will not mix to form a solution. Oil will easily dissolve in gasoline but will not dissolve in water because of the difference in polarity. There is a general rule of solubility (and hence cleaning) that says like dissolves like.

For a substance to play a role in cleaning, it must remove items such as dirt and grease from various materials (e.g. clothes, carpets, surfaces, etc.). Dirt and grime usually adhere to skin, clothing, and other types of surfaces because they combine with greases and oils that act similarly to glues. Because oil is not hydrophilic and thus not soluble in water, attempting to wash dirt and grime from surfaces using water alone results in little success.

To understand the physical process involved with effective cleaning, it is helpful to understand basic soap and detergent chemistry. One of the most common liquids used for cleaning is water. Water (H2O) has a property called surface tension. Within/ throughout the bulk of the liquid water, the total force of attraction exerted by any one molecule of water on all its surrounding neighbours is dispersed spherically, in all directions. However, at the surface, a tension is created as the water molecules at the surface are pulled into the main body/ bulk of the liquid water. It is this strong attraction of the surface water molecules for each other and for the water molecules immediately below them that results in the cohesive property of the surface known as surface tension. Surface tension causes water to bead up on surfaces (e.g. human skin, glass, cloth fabric, furniture), which shows the overall wetting of the surface and thereby inhibits the cleaning process. For the cleaning process to be successful, the surface tension must be reduced so that water can evenly overspread, wet, and saturate surfaces. Organic (carbon based) chemicals that decrease surface tension are termed surface active agents or simply surfactants. When added to water, surfactants can reduce the surface tension to only 30% of that of water molecules. Overall, the presence of surfactants within a body of water significantly reduces the surface tension normally exhibited by the water.

Surfactants are also involved with other aspects of successful cleaning, including loosening, emulsifying (dispersing one liquid into another immiscible liquid, which prevents oily soils from resettling on the surface of water), and holding soil in suspension until it can be removed and/ or washed away. Surfactants may act as emulsifiers, substances that help to form and stabilize emulsions. This means that, although oil, which attracts greasy dirt, does not actually mix with the water, soap can suspend the oil and greasy dirt in such a way that it can be removed. Therefore, the cleaning power of many surfactants results from the enhanced ability of the water to wet the normally hydrophilic (non water soluble) surface, penetrate into the fibers more freely, and lift off the dirt. For this reason, surfactants are often referred to as wetting agents because they help water actually wet surfaces. Many surfactants provide an alkaline environment within the cleaning process, which can be essential in removing acid-based soils and grime.

Surfactants are classified by their ionic (electrically charged particles) properties in water. Surfactant molecules can be described as resembling a tadpole because they contain a fairly long fatty acid tail (hydrophobic or water insoluble) and a small, often electrically charged head (hydrophilic or water soluble). The long hydrocarbon (CH2 groups) tails or surfactants are soluble in hydrophobic substances such as oil, and the hydrophilic heads of surfactants containing the carboxyl or sulfonate groups are soluble in water. Water is polar: the H2O molecules have an attraction for other polar substances, such as common table salt (NaCl). When salt is added to water, the salt molecule ions are attracted to, and become surrounded by, the water molecules. This occurrence is known as solubility. Oil, however, is a non polar substance and therefore will only dissolve in other non polar substances. Because non polar substances cannot form hydrogen bonds with water molecules, they do not tend to bind well with water and are essentially insoluble in water. Thus, there is an actual chemical foundation behind the phenomenon that oil and water do not mix! Surfactants promote an environment in which oil can seemingly dissolve in water by bridging the oil water interface.

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